1. Field of the Invention
The field of the invention relates to anti-scatter grids used in medical imaging by means of radiation.
2. Description of the Prior Art
Anti-scatter grids are widely used in medical imaging to improve image contrast quality.
FIG. 1 illustrates a conventional medical imaging system.
Such a system comprises a radiation source 1, for example of the X-ray type, which projects a conical beam for illuminating an object 2, such as for example a part or an organ of a patient to be examined before reaching an image receptor 3 comprising a detector array.
An anti-scatter grid 4 is inserted between the source 1 and the image receptor 3 so as to attenuate the radiation not coming directly from the source 1, particularly scattered radiation.
Conventionally, an anti-scatter grid comprises an alternation of thin plane strips consisting of a highly X-ray absorbent material, such as lead for example, separated by spacer bands that are very transparent to said radiation. The document U.S. Pat. No. 7,356,126 describes such a grid.
The grid may further be focussed according to the terminology defined by the standard IEC 60627 relating to X-ray diagnostic imaging devices.
Focussed grids make it possible to improve effective ray transmission.
FIG. 2 illustrates the general principle of a focussed grid.
In use, the strips 20 of material having high X-ray absorption properties (radio-opaque strips) are distributed and oriented towards the focal point formed by the radiation source. All the planes of the strips intersect along the same line D. The grid is used by positioning the focal point F of the X-ray source 1 substantially on the line D.
One problem with the grids as described above is that the regularity thereof is not sufficient, particularly in applications where the grid is fixed in relation to the image receptor. Furthermore, these grids are difficult to produce.
In order to remedy this production problem, a process for producing an anti-scatter grid has been developed.
The document U.S. Pat. No. 5,557,650 describes such a process. Such a process implements a substrate having low X-ray absorption properties wherein grooves are formed. Once the grooves have been formed, said grooves are filled with a material 31 having high X-ray absorption properties.
FIG. 3 illustrates a cross-section of a grid obtained with this process. The grid obtained comprises a substrate S comprising a plurality of grooves 30 in the thickness of the substrate. In this grid, the grooves 30 are filled with a material 31 having X-ray absorption properties. The grooves only open onto one face of the substrate. Furthermore, the orientation 32 of the grooves is such that the grid is focussed on the focal point of the X-ray source.
Such a grid is arranged in an imaging system such that the face 33 where the grooves open is on the side of the X-ray source. The other face 34 is arranged on the side of the image receptor.
One problem is that, in use, it has been observed that the substrate produced diffuse radiation SG which was not stopped by the strips obtained by filling the groove such that the primary radiation Sp passing through the substrate is degraded by the radiation SG scattered by the substrate. In use, the performances of such a grid are limited.